The present invention relates to a drill bit and particularly to a drill bit usable for deep hole drilling. The drill bit is particularly efficient in delivering coolant to its cutting tip and removing swarf and other debris from the cutting areas.
Various problems associated with drilling deep holes particularly in hard materials such as high temperature stainless steels and aircraft specification No. steels are described in British Patent specification 1 383 090. Briefly summarized, the problems involve keeping the cutting tip of the drill at a reasonable temperature and assuring that the cutting area is kept free of swarf generated by the drilling operation by efficient removal of the swarf and other debris therefrom. For the reasons explained in the foregoing British specification the use of "gun drills", which are expensive and employ high pressure coolant pumps is not practical for many applications.
The foregoing British patent provides a drill which comprises a tubular shank of flattened cross-section which defines one or more passageways for coolant fluid. A flat blade with a drilling tip is positioned at one end of the shank, the blade being oriented not to block the coolant passageways in the shank. Coolant supplied through the passageways impinges on the drilling tip and the cutting area and flows out of the drilled hole along the exterior flattened periphery of the shank, carrying swarf with it. The shank is fashioned from a circular tube which is longitudinally flattened to produce an approximate figure-of-8 shaped cross-section, defining two passageways separated by contacting wall portions of the flattened tube.
Although the above-described bit has advanced the art of drill making, several distinct disadvantages remain. For example, the flattened shank is not sufficiently sturdy, and at greater lengths it is susceptible to buckling and/or to twisting. Further, the flat blade causes coolant to wash over the outside of the blade tending to wash the swarf forward into the cutting area prior to evacuation. This results in less efficient swarf removable and could lead to swarf blockage. Moreover, the flattened shank which is not as hard as the cutting blade is exposed to a major proportion of the force produced by the fast moving swarf. As a result, erosion weakens the braze connection between the blade and the shank and eventual outright breakage will occur. The flat blade provides relatively little control over the size of the bore and its concentricity and, in addition, a less than optimal hole surface finish is obtained. Finally, the blade lacks chip breakers to chip the material being drilled to produce swarf which is not stringy and therefore more efficiently removable.